The instant invention relates to an echo tracking system for apparatus for measuring the position of a mobile wall.
The invention is useful in all fields where it is desired to follow the movement, over a period of time, of the position of a moving wall and in particular in the medical field. In the latter case, the invention may be employed to follow the changes over a period of time of the position of an interface between two tissues and as an example the depth of the anterior and posterior walls of a blood vessel to determine changes over a period of time in the diameter of this blood vessel.
FIG. 1 illustrates schematically the known principle of the measurement of the position of a mobile wall. This figure represents an ultrasonic wave transducer 2 placed on the skin 4 of a subject opposite a radial artery 6 shown in cross section. The transducer 2 is so controlled by an electronic circuit that it emits an ultrasonic wave pulse 8 and receives the echoes resulting from reflection of this pulse from the interfaces artery-tissue or artery-blood. Depending on the frequency of the ultrasound transducer, one can detect four distinct echoes 10, 12, 14, 16 or only two echoes corresponding to a combination of the echoes 10 and 12 and a to combination of the echoes 14 and 16 respectively.
The movement of an interface is determined in the following manner. The transducer 2 emits a pulse 8 with a repetition rate generally between 100 Hz and 20 kHz. In order to follow the position of the echo, whose delay after the pulse 8 depends on the position of the interface, a time window is used the width of which is fixed so as to define a time interval in which the echo is expected and which is opened with a delay which is adjustable after transmission of the pulse 8. This delay is adjusted after each cycle in a way so that the echo is detected in the center of this window if the interface is stationary.
Knowledge of the position of each interface as a function of time makes it possible, by noting the changes, to determine the changes in the diameter of the blood vessel 6 as a function of time.
An echo tracking system is described in particular in the article "A phase-locked echo tracking system for recording arterial diameter changes in vivo" by D. E. Hokanson et al., published in the Journal of Applied Physiology, Vol. 32, No. 5, p. 728-733, 1972. This echo tracking system is essentially an analog system which limits its accuracy. More recently, digital echo tracking systems have been proposed. One such echo tracking system in particular is described in the article "A digital technique for tracking moving interfaces" by D. H. Groves et al., published in Ultrasound Med. & Biol., Vol. 8, No. 2, p. 185-190, 1982. FIG. 2 shows a circuit illustrating the structure of this digital echo tracking system and FIG. 3 shows a timing diagram illustrating the functioning of this circuit.
This echo tracking system comprises a logic circuit 18, a depth counter 20, an enabling counter 22, a digital comparator and a register 26. The logic circuit 18 receives a pulse signal A synchronous with the excitation signal applied to the transducer, a signal VAL delivered by the comparator 24 to indicate the beginning of the time window, and an ECHO signal which represents the echo received by the ultrasonic transducer after shaping and digitization. The logic circuit 18 includes a generator producing a clock signal CLK which is used to measure the echo delay over the pulse A and to clock the counters 20, 22.
FIG. 3 illustrates the functioning of this echo tracking system for two consecutive cycles n and n+1. Shortly before the beginning of the cycle n, the counter 22 is stored with a value C.sub.1,n which, as will be seen below, is equal to the contents of the counter 20 at the end of the cycle n-1. This value is such that in counting down at the rate of the clock signal CLK the contents of the counter is equal to zero at the instant corresponding to the middle of the time window if the wall is stationary. At the moment when the time window has to be open the counter 22 thus has a value .delta. which is not equal to zero. To determine the beginning of the time window it is thus sufficient to compare the contents of the counter 22 to the fixed value .delta.. This is effected by the comparator 24 which receives on the one hand the value contained in the counter 22 and, on the other hand, the value .delta. memorized in the register 26.
As regards the counter 20, its content is set at zero shortly before the cycle n. Counting is stopped by the logic circuit 18 on reception of the echo C. The counter 20 then stores a value C.sub.2,n which represents the position of the mobile wall. This value is used in the following cycle n+1 to clamp again the position of the time window. This is effected in two stages: The content of the counter 20 is transferred into the counter 22, and the counter 20 is then reset.
It is known that echo tracking systems handle very high frequency digital signals. In the known echo tracking system shown in FIG. 2, the transducer transmits 3.5 MHz pulses at a repetition rate of 12.5 kHz and the clock signal CLK has a frequency of 20 MHz.
It follows that the echo tracking system shown in FIG. 2 has a high power consumption since the two counters 20, 22 and the comparator 24 are each in operation for a large part of each cycle. This is an obstacle against the production of a portable apparatus due to the considerable weight of the battery or accumulator needed if the power independency of the apparatus is to be reasonable.
It is moreover evident that there is a need for portable apparatus in fields such as the medical field. It would, indeed, be easier for a doctor to transport the apparatus from one room to another in a hospital to examine the patients rather than to have to move the patients to a fixed apparatus. A portable apparatus would also enable the doctor to examine a patient at home. Finally, a sufficiently small portable apparatus could be worn directly by the user, for example in the form of a wrist watch.
Another disadvantage of the known apparatus shown in FIG. 2 lies in its high cost which is in part due to the use of a very fast comparator. This is a bar to the industrial development of this type of apparatus.